29th Jul 2009, 20:01
The various fixes from ten+ years ago appear to have stopped the "uncommanded rudder" yaw upset events suffered by the Boeing 737-100/500 during the 1980's and 1990's.
They were sequential, not concurrent, therefore someone has surely worked backward to discern which one or more was/were effective.
It certainly wasn't the dual-concentric hydraulic valve replacement as the US NTSB suspected.
We passengers are relieved that the fixes worked, however, let us have a discussion of which for educational purposes.
Question posed, about post-accident design changes to Fin-Rudder System:“… various fixes … stopped the "uncommanded rudder" yaw upset events suffered by the Boeing 737 … [fixes] … were sequential … someone … discern which … [fix] was/were effective…. wasn't the dual-concentric hydraulic valve replacement …”
Here's a summary of AD's [fixes]
Lessons Learned From Transport Airplane Accidents (http://accidents-ll.faa.gov/ll_main.cfm?TabID=1&LLID=1&LLTypeID=11)
Citing such a single definitive “fix” would be unusual for cases involving complex failure interactions.
During the past eight decades of mysterious inflight upsets and investigations, some cases’ causes were more “probable” than in other cases.
The investigators’ final “Probable Cause” often lacks complete certainty, and their final P.C. also may avoid definite details of the specific failure interactions.
Your question focused on a later model and 1990’s upsets, but the exemplars for those B737 “discrepant rudder” events were cases investigated decades earlier.
Later engineers would direct you to study the investigation of AA “Flight One” / 1Mar62, fatal accident of a B707-123B, Idlewild. The initial upset included Yaw x Roll coupling, and progressed to a near vertical dive at impact. Earlier engineers would point you toward the earlier case: B307 “Stratoliner” / 18Mar39 yaw x roll, dive, inflight break-up. Note that the investigators offered their Probable Causes. After the initial 1939 Stratoliner investigation, later the manufacturer’s own engineers presented further details about design-errs in the initial design of Stratoliner’s Fin-Rudder.
An example of a very specific PC was WAL366 /31Mar71, B720, at ONT: Low altitude upset, during crew training. From the AAR 72-18: "...while rotating to the right about its roll and yaw axes ... the nose of the aircraft descended to a near vertical downward position."
“… the probable cause of this accident was the failure of the aircraft rudder hydraulic actuator support fitting. The failure of the fitting resulted in the inapparent loss of left rudder control which, under the conditions of the flight, precluded the pilots’ ability to maintain directional control …”
Then, due to complexity of such a directional control system, the investigators added various factors that contributed to some uncertainty: “… loss of rudder control was not apparent to the crew due to the continued presence of the artificially provided ‘rudder feel’ …” [Finding #15].
In contrast, consider the less certain PC from AA Flight One / 1Mar62, B707: probable cause: "...rudder control system malfunction producing yaw, sideslip, and roll leading to a loss of control from which recovery was not effective."
And consider the revised PC from UA585 / 3Mar91, B737-200 at COS, and note some uncertainty expressed by their words “most likely”:“… the probable cause of the United Airlines flight 585 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.”
12th Aug 2009, 01:40
IGh, see also:
B737 Rudder Epilog - JetPhotos.Net Forums - The Friendly Way to Fly (http://forums.jetphotos.net/showthread.php?t=48087) :)